Simple version of the promise

function MyPromise(fn) { const pending = 'pending', resolved = 'resolved', rejected = 'rejected'; const that = this; const value = null; const resolvedCallbackList = [], rejectCallbackList = []; that.value = value; that.statusMap = { pending, resolved, rejected } that.status = pending; that.resolvedCallbackList = resolvedCallbackList; that.rejectCallbackList = rejectCallbackList; function resolve(v) { if (that.status === pending) { if (v instanceof MyPromise) { return v.then(resolve, reject) } that.value = v; that.status = resolved; setTimeout(() => { that.resolvedCallbackList.forEach(f => { try { f(v) } catch(e) { reject(e) } }) }) } } function reject(v) { if (that.status === pending) { that.value = v; that.status = rejected; setTimeout(() => { that.rejectCallbackList.forEach(f => { try { f(v) } catch(e) { reject(e) } }) }) } } try { fn(resolve, reject) } catch(e) { reject(e) }} MyPromise.prototype.then = function(onFulfilled = v => v, onRejected = err => err) { const that = this; let newPromise; if (that.status === that.statusMap.pending) { newPromise = new MyPromise(function(resolve, reject) { that.resolvedCallbackList.push(() => { resolve(onFulfilled(that.value)) }); that.rejectCallbackList.push(() => { reject(onRejected(that.value)) }); })}; if (that.status === that.statusMap.resolved) { newPromise = new Promise(function(resolve, reject) { resolve(onFulfilled(that.value)) }) } if (that.status === that.statusMap.rejected) { newPromise = new Promise(function(resolve, reject) { reject(onRejected(that.value)) }) } return newPromise; } MyPromise.all = function promiseAll(promises) { return new MyPromise(function(resolve, reject) { var resolvedCounter = 0; Var promiseNum = promises. Length; var resolvedValues = new Array(promiseNum); for (let i = 0; i < promiseNum; i++) { promises[i].then(function(value) { resolvedCounter++ resolvedValues[i] = value if (resolvedCounter == promiseNum) Resolve (resolvedValues)}}, function(reason) { return reject(reason) }) } })} MyPromise.resolve = function(v) { return new MyPromise(function(res, rej) { res(v) })}Copy the code